EP2816131A1 - Acier pour tiges laminées pour forgeage à chaud, matériau grossièrement façonné forgé à chaud et rail commun, et leur procédé de fabrication - Google Patents

Acier pour tiges laminées pour forgeage à chaud, matériau grossièrement façonné forgé à chaud et rail commun, et leur procédé de fabrication Download PDF

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Publication number
EP2816131A1
EP2816131A1 EP13749063.7A EP13749063A EP2816131A1 EP 2816131 A1 EP2816131 A1 EP 2816131A1 EP 13749063 A EP13749063 A EP 13749063A EP 2816131 A1 EP2816131 A1 EP 2816131A1
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Prior art keywords
steel bar
rolled steel
hot
section material
less
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EP13749063.7A
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German (de)
English (en)
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EP2816131B1 (fr
EP2816131A4 (fr
Inventor
Naoki Matsui
Masashi Higashida
Hitoshi Matsumoto
Yutaka Neishi
Taizo Makino
Kouji Morita
Yoshihiro Tanimura
Naoyuki Sashima
Toshimasa ITOU
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Denso Corp
Nippon Steel Corp
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Denso Corp
Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21K—MAKING FORGED OR PRESSED METAL PRODUCTS, e.g. HORSE-SHOES, RIVETS, BOLTS OR WHEELS
    • B21K3/00—Making engine or like machine parts not covered by sub-groups of B21K1/00; Making propellers or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02M—SUPPLYING COMBUSTION ENGINES IN GENERAL WITH COMBUSTIBLE MIXTURES OR CONSTITUENTS THEREOF
    • F02M63/00—Other fuel-injection apparatus having pertinent characteristics not provided for in groups F02M39/00 - F02M57/00 or F02M67/00; Details, component parts, or accessories of fuel-injection apparatus, not provided for in, or of interest apart from, the apparatus of groups F02M39/00 - F02M61/00 or F02M67/00; Combination of fuel pump with other devices, e.g. lubricating oil pump
    • F02M63/02—Fuel-injection apparatus having several injectors fed by a common pumping element, or having several pumping elements feeding a common injector; Fuel-injection apparatus having provisions for cutting-out pumps, pumping elements, or injectors; Fuel-injection apparatus having provisions for variably interconnecting pumping elements and injectors alternatively
    • F02M63/0225—Fuel-injection apparatus having a common rail feeding several injectors ; Means for varying pressure in common rails; Pumps feeding common rails
    • F02M63/0275—Arrangement of common rails
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02M—SUPPLYING COMBUSTION ENGINES IN GENERAL WITH COMBUSTIBLE MIXTURES OR CONSTITUENTS THEREOF
    • F02M2200/00—Details of fuel-injection apparatus, not otherwise provided for
    • F02M2200/90—Selection of particular materials
    • F02M2200/9053—Metals
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02M—SUPPLYING COMBUSTION ENGINES IN GENERAL WITH COMBUSTIBLE MIXTURES OR CONSTITUENTS THEREOF
    • F02M55/00—Fuel-injection apparatus characterised by their fuel conduits or their venting means; Arrangements of conduits between fuel tank and pump F02M37/00
    • F02M55/02—Conduits between injection pumps and injectors, e.g. conduits between pump and common-rail or conduits between common-rail and injectors
    • F02M55/025—Common rails

Definitions

  • the present invention relates to a rolled steel bar for hot forging, a hot-forged section material, a common rail, and a method for producing the common rail. More particularly, it relates to a rolled steel bar for hot forging suitable as a starting material for a common rail used for a diesel engine fuel injection system, a hot-forged section material produced by forming the rolled steel bar, the common rail, and a method for producing the common rail.
  • a common rail is a hollow shaped part that is used for the diesel engine fuel injection system and temporarily stores the pressurized fuel before the fuel is injected into the engine.
  • a steel material used for the common rail is required to have a high fatigue strength against the internal pressure, to have a high fracture toughness to prevent brittle fracture even if a fatigue crack is generated by the repeatedly applied internal pressure, to have high machinability to facilitate the formation of a plurality of intersecting holes formed in the part, and so on.
  • a non-thermally refined steel material in which a steel bar produced by hot rolling (hereinafter, a steel bar as is hot-rolled, which steel bar is produced by hot rolling is referred to as a "rolled steel bar”) is formed by hot forging (hereinafter, a rolled steel bar as is formed by hot forging is referred to as a "hot-forged section material”), and a desired strength can be obtained without performing heat treatment of quenching and tempering, that is, "thermal refining treatment".
  • Patent Document 1 discloses a free cutting steel that contains Bi and S as inclusion forming elements, and is provided with both of high fatigue strength and excellent machinability, and a fuel injection system part using the free cutting steel.
  • Patent Document 2 discloses a steel for common rail excellent in fatigue properties, in which REM is contained, and the dispersion mode of sulfide-based inclusions, nitride-based inclusions, and oxide-based inclusions is controlled, and a common rail.
  • Patent Document 3 discloses a steel-made high-strength fabricated product excellent in shock resistance and balance of strength-ductility, in which a steel material containing a proper amount of one or more elements selected from a group consisting of Nb, Ti and V and a proper amount of A1 is used, and the metal micro-structure of the steel material is made to consist of ferrite, retained austenite, and bainite and/or martensite by controlling the cooling after hot forging.
  • Patent Document 4 discloses a steel excellent in fatigue properties in which the length-to-width ratio of Mn sulfide-based inclusion is made a certain value or lower, and a steel part produced from the steel.
  • Patent Document 5 discloses a ferrite/pearlite type non-thermally refined steel for hot forging, in which the contents of C, S and V are especially controlled, and the fatigue strength and the cutting workability using a cemented carbide drill are excellent, and a common rail using the non-thermally refined steel.
  • Non-Patent Document 1 Akira Suzuki, Takeshi Suzuki, Yutaka Nagaoka, and Yoshihiro Iwata: On Space Between Secondary Dendrite Arms of Carbon Steel Different in Carbon Content, Journal of the Japan Institute of Metals, 32 (1968), pp. 1301-1305
  • Patent Document 2 the steel must contain expensive alloying elements such as Bi and REM to improve the machinability, so that the cost increases.
  • the thermal refining treatment leads to a further increase in cost.
  • the steel contains one element or two or more elements of Mg, Ca, Zr, Te, and REM. Therefore, the cost of alloying elements contained in the starting material increases. Also, coarse oxides sometimes exist in the steel, so that an excellent fatigue strength cannot necessarily be attained.
  • an objective of the present invention is to provide a rolled steel bar for hot forging capable of being produced at a low cost, which steel bar is excellent in fatigue strength, fracture toughness value, and machinability without being subjected to thermal refining treatment, and is suitable as a starting material for a common rail for a fuel injection system used at a high injection pressure, a hot-forged section material produced by hot-forging the rolled steel bar, and a method for producing the common rail using the section material.
  • the common rail for a fuel injection system used at a high injection pressure is produced by the method described below.
  • a rolled steel bar which is a starting material
  • the rolled steel bar is formed into a hot-forged section material by pressing down the rolled steel bar in the direction perpendicular to the rolling direction of the rolled steel bar due to hot forging.
  • a through hole is formed in the center axis direction (the rolling direction of the rolled steel bar, which is a starting material) of the center part of the transverse cross section thereof by cutting work using a drill, and minute holes are also formed by cutting work so as to intersect with the through hole.
  • the common rail In the interior of common rail in which the through hole has been formed in the center part, the pressure accumulation (pressurizing) and pressure discharge (depressurizing) of fuel are repeated at a high pressure. Therefore, a tensile stress acts repeatedly in the circumferential direction of the inner surface of the through hole of common rail. Accordingly, the common rail is required to have a high fatigue strength against the stress in the direction perpendicular to the center axis of common rail (hereinafter, the fatigue strength against the stress in the direction perpendicular to the center axis is referred to as a "transverse fatigue strength").
  • the hot-forged section material is produced by pressing down and forming the rolled steel, which is a starting material, in the direction perpendicular to the rolling direction of the rolled steel bar as described above, the sizes and distribution state of nonmetallic inclusions in the rolled steel bar, which inclusions have been elongated in the rolling direction due to hot rolling, are transferred to the hot-forged section material almost as they are. Therefore, for the common rail formed with the through hole in the center part of the hot-forged section material, the nonmetallic inclusions elongated in the direction parallel with the center axis (the rolling direction of the rolled steel bar, which is a starting material) are distributed, so that the transverse fatigue strength tends to decrease.
  • the transverse fatigue strength has to be enhanced in the state of the hot-forged section material before the through hole and minute holes are formed.
  • the tensile strength of the hot-forged section material has to be high.
  • the machinability is decreased in the cutting process in which the hot-forged section material is cut in a non-thermally refined state. As a result, the cutting cost rises, and also the cutting time is prolonged.
  • the non-thermally refined hot-forged section material in which the tensile strength is enhanced for increasing the transverse fatigue strength has a tendency for the fracture toughness value to decrease. If the fracture toughness value is low, brittle fracture may occur in the case where a fatigue crack is generated by the internal pressure repeatedly applied in the interior of common rail. For the hot-forged section material, therefore, both of the tensile strength and the fracture toughness value has to be high.
  • the present inventors examined in detail the relationship between the chemical composition, micro-structure, and sizes and distribution of nonmetallic inclusions of the steel material and the transverse fatigue strength, fracture toughness value, and machinability. As the result, the present inventors came to obtain the following findings.
  • the present invention has been accomplished on the basis of the above-described findings, and involves the rolled steel bar for hot forging, the hot-forged section material, the common rail, and the method for producing the common rail described below.
  • impurities means components that are mixed in from raw materials such as ore and scrap, production environments, and the like when the steel is produced on an industrial basis.
  • a non-thermally refined hot-forged section material excellent in transverse fatigue strength, fracture toughness value, and machinability can be obtained. Also, by forming intersecting holes in the hot-forged section material of the present invention, a common rail for a fuel injection system used at a high injection pressure can be produced at a low cost.
  • C carbon
  • C is an element for strengthening a steel, and therefore 0.25% or more of C has to be contained.
  • the content of C is set to 0.25 to 0.50%.
  • the C content is preferably 0.29% or more, and preferably 0.45% or less.
  • Si is a deoxidizing element, and also is an element necessary for strengthening ferrite by means of solid-solution strengthening and for enhancing the tensile strength after hot forging. In order to achieve these effects, 0.40% or more of Si has to be contained. On the other hand, if the content of Si is more than 1.0%, not only the effects are saturated, but also decarburization of the surfaces of the rolled steel bar for hot forging and non-thermally refined hot-forged section material becomes remarkable. Therefore, the content of Si is set to 0.40 to 1.0%. The Si content is preferably 0.45% or more, and preferably 0.80% or less.
  • Mn manganese
  • Mn manganese
  • Mn is an element necessary for strengthening ferrite by means of solid-solution strengthening and for enhancing the tensile strength after hot forging, and therefore 1.0% or more of Mn has to be contained.
  • the content of Mn is set to 1.0 to 1.6%.
  • the Mn content is preferably 1.1% or more, and preferably 1.4% or less.
  • S is an important element in the present invention. Sulfur combines with Mn to form sulfides. In particular, if a large number of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m exist in the rolled steel bar, an effect of suppressing the growth of austenite grains in hot forging is achieved. Therefore, if the number density of fine sulfides is increased, the structure of hot-forged section material is refined, and the fracture toughness value can be increased. Furthermore, the machinability is improved by sulfides. In order to achieve these effects, 0.005% or more of S must be contained.
  • the content of S is set to 0.005 to 0.035%.
  • the S content is preferably 0.010% or more, and preferably less than 0.030%, further preferably 0.025% or less.
  • Al (aluminum) has functions of not only a deoxidizing, but also suppressing the growth of austenite grains during hot forging due to the pinning effect by combining with N to form fine AlN. Therefore, Al has an effect of making the structure of hot-forged section material fine, and increasing the fracture toughness value. For this purpose, 0.005% or more of Al has to be contained. On the other hand, if the content of Al is more than 0.050%, the effects thereof are saturated. Therefore, the content of Al is set to 0.005 to 0.050%.
  • the Al content is preferably 0.010% or more, and preferably 0.040% or less.
  • V vanadium
  • V vanadium
  • the V content is preferably 0.14% or more, and preferably 0.29% or less.
  • N nitrogen
  • N has a function of enhancing the transverse fatigue strength of non-thermally refined hot-forged section material by combining with V to form fine nitrides or carbonitrides.
  • N combines with Al to form fine AlN to suppress the growth of austenite grains during hot forging due to the pinning effect. Therefore, N has an effect of refing the structure of hot-forged section material, and increasing the fracture toughness value. For this purpose, 0.005% or more of N has to be contained. However, if the content of N is more than 0.030%, pinholes are sometimes formed in the steel. Therefore, the content of N is set to 0.005 to 0.030%.
  • the N content is preferably 0.008% or more, and preferably 0.020% or less.
  • the chemical composition of the rolled steel bar for hot forging and the hot-forged section material of the present invention consists of the above-described elements ranging from C to N, the balance being Fe and impurities.
  • impurities means components that are mixed in from raw materials such as ore and scrap, production environments, and the like when the steel is produced on an industrial basis.
  • P phosphorus
  • the content of P is set to 0.035% or less.
  • the P content is preferably 0.030% or less. Also, it is desirable to set the content of P contained as an impurity as low as possible as far as the cost of steel-making process is not raised.
  • O oxygen
  • a deoxidizing element such as Al, Si
  • a coarse oxide serves as a starting point of fatigue fracture, and decreases the transverse fatigue strength of non-thermally refined hot-forged section material.
  • the existence of oxide having a great width causes a decrease in transverse fatigue strength.
  • the content of O is set to 0.0030% or less.
  • the O content is preferably 0.0015% or less. Also, it is desirable to set the content of O contained as an impurity as low as possible as far as the cost of steel-making process is not raised.
  • Another feature of the rolled steel bar for hot forging and the hot-forged section material of the present invention is to contain one or more elements selected from (a) Ti, and (b) Cu, Ni, Cr and Mo, each having a content described below, in lieu of a part of Fe.
  • Ti titanium
  • Ti has an effect of suppressing the growth of austenite grains by combining with N to form TiN. Therefore, Ti makes the structure of hot-forged section material fine, and can increase the fracture toughness value.
  • Ti may be contained as necessary. However, if the content of Ti is more than 0.030%, the precipitation strengthening due to Ti carbides is remarkable, and thereby the fracture toughness value may be decreased. Therefore, the content of Ti, if being contained, is set to 0.030% or less.
  • the Ti content is preferably 0.020% or less. In order to steadily achieve the above-described effects, it is preferable to contain 0.002% or more of Ti. Further preferably, 0.004% or more of Ti is contained.
  • Cu copper
  • Cu is an element for strengthening a steel by means of solid-solution strengthening, and therefore Cu may be contained as necessary.
  • the content of Cu is set to 0.30% or less.
  • the Cu content is preferably 0.20% or less. In order to steadily achieve the above-described effect, it is preferable to contain 0.03% or more of Cu. Further preferably, 0.05% or more of Cu is contained.
  • Ni nickel
  • Ni is an element for strengthening a steel by means of solid-solution strengthening, and therefore Ni may be contained as necessary.
  • the content of Ni is set to 0.20% or less.
  • the Ni content is preferably 0.10% or less. In order to steadily achieve the above-described effect, it is preferable to contain 0.03% or more of Ni. Further preferably, 0.05% or more of Ni is contained.
  • Cr Cr
  • Cr Cr
  • Cr chromium
  • the content of Cr is set to 0.50% or less.
  • the Cr content is preferably 0.30% or less. In order to steadily achieve the above-described effect, it is preferable to contain 0.03% or more of Cr. Further preferably, 0.05% or more of Cr is contained.
  • Mo mobdenum
  • Mo is an element for strengthening a steel by means of solid-solution strengthening. Therefore, in the case where it is desired to enhance the tensile strength, Mo may be contained. However, if the content of Mo is more than 0.10%, not only the effect thereof is saturated, but also the hardenability is enhanced, and bainite is formed undesirably after hot forging, whereby the fracture toughness value and machinability may be decreased. Therefore, the content of Mo, if being contained, is set to 0.10% or less.
  • the Mo content is preferably 0.08% or less. In order to steadily achieve the above-described effect, it is preferable to contain 0.02% or more of Mo. Further preferably, 0.04% or more of Mo is contained.
  • Only one element of Cu, Ni, Cr and Mo can be contained, or two or more elements selected from these elements can be contained compositely.
  • the total amount in the case where these elements are contained compositely is preferably 0.60% or less.
  • Fn1 is a parameter that is represented by the following Formula (i), and affords an index of the influence exerted on tensile strength.
  • Formula (i) For the hot-forged section material obtained by hot forging using the rolled steel bar for hot forging, in order to assure a high tensile strength of 900 MPa or higher even in the case where the ratio of ferrite in the ferrite/pearlite structure is increased, the content of each element has to be controlled so that the value of Fn1 is within the defined range. If the value of Fn1 is smaller than 0.90, the tensile strength of the non-thermally refined hot-forged section material decreases, so that a desired transverse fatigue strength cannot be attained. Therefore, the value of Fn1 has to be set to 0.90 or larger.
  • the value of Fn1 is preferably 0.95 or larger. On the other hand, if the value of Fn1 is larger than 1.20, there is a possibility that bainite may be formed in the hot-forged section material after hot forging. If bainite is formed, the fracture toughness value and machinability of the hot-forged section material are decreased. Therefore, the value of Fn1 is set to 1.20 or smaller. The value of Fn1 is preferably 1.16 or smaller.
  • Fn ⁇ 1 C + Si / 10 + Mn / 5 + 5 ⁇ Cr / 22 + 1.65 ⁇ V - 5 ⁇ S / 7 where, the symbol of an element in Formula (i) represents the content (mass%) of the element.
  • the predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing by taking the width of nonmetallic inclusion in an R 1 /2 part (R 1 : radius of rolled steel bar) of a longitudinal cross section, and in an R 2 /2 part (R 2 : radius of section material) or in a T/4 part (T: thickness of section material) of a longitudinal cross section as W ( ⁇ m) is 99.99% is made 100 ⁇ m or narrower.
  • the predicted maximum width of nonmetallic inclusions at the time when a cumulative distribution function obtained by extreme value statistical processing is 99.99% can be determined by the method described below.
  • explanation is given of the case of the rolled steel bar for hot forging only. The same is true for the case of the hot-forged section material.
  • Ten test specimens each measuring 5 mm wide ⁇ 15 mm long are cut out so that the longitudinal cross section including the R 1 /2 part of the rolled steel bar for hot forging is a surface to be inspected, and thereafter is mirror polished. The polished surface is made the surface to be inspected. Subsequently, by making the area to be inspected of one visual field 2.954 mm 2 , which is a range observed under an optical microscope having a magnification of ⁇ 100, five visual fields per one test specimen, that is, a total of 50 visual fields are observed, and the width W ( ⁇ m) of inclusion having the maximum width of the nonmetallic inclusions observed in each visual field is measured.
  • the common rail is formed by pressing down the rolled steel bar for hot rolling in the direction perpendicular to the rolling direction of the rolled steel bar.
  • the hot-forged section material formed by pressing down the rolled steel bar in this direction the sizes and distribution state of nonmetallic inclusions in the rolled steel bar, which inclusions have been elongated in the rolling direction due to hot rolling, are transferred almost as they are. Therefore, the transverse fatigue strength in the hot-forged section material is affected by the predicted maximum width of nonmetallic inclusions of the rolled steel bar.
  • the nonmetallic inclusions mean oxides, sulfides, and nitrides existing in a steel.
  • the nonmetallic inclusions of the rolled steel bar are elongated by hot rolling, and are cut, so that the widths thereof are decreased. If a nonmetallic inclusion having a great width exists in the rolled steel bar, the transverse fatigue strength of the hot-forged section material is decreased.
  • the predicted maximum width of nonmetallic inclusions of the rolled steel bar which is obtained by extreme value statistical processing, can be decreased, for example, by the method described below.
  • Coarse oxides consisting mainly of Al 2 O 3 can exist in the steel with a certain probability. Since oxides agglomerate in the molten steel, being formed into clusters, and are coarsened, oxides are removed sufficiently at the stage of refining. Further, the oxides agglomerating at the refining stage are removed and solidified to form a cast piece or an ingot. The cast piece or ingot turns finally to the rolled steel bar for hot forging through a process of steel bar rolling or blooming and steel bar rolling.
  • a total reduction ratio represented by the ratio between both the cross-sectional areas, that is, S O /S F is made 40 or higher.
  • the oxides, sulfides, and nitrides are elongated or cut, so that the predicted maximum width of nonmetallic inclusions of the rolled steel bar can easily be made smaller than 100 ⁇ m.
  • the upper limit of reduction ratio is preferably set to 600.
  • fine sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m exist at a predetermined number density in the rolled steel bar for hot forging, there is achieved an effect of suppressing the growth of austenite grains during hot forging due to the pinning effect of crystal grain boundary.
  • the sulfides each having a circle-equivalent diameter of smaller than 0.3 ⁇ m are dissolved by heating during hot forging, so that there is a possibility that the pinning effect cannot be achieved sufficiently.
  • the sulfides each having a circle-equivalent diameter of 1.0 ⁇ m or larger a remarkable pinning effect of crystal grain boundary cannot be anticipated.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m is lower than 500 pieces/mm 2 , the pinning effect of crystal grain boundary is insufficient, and the structure after hot forging is coarse, whereby the fracture toughness value of the hot-forged section material may be decreased. Therefore, in the rolled steel bar for hot forging according to the present invention, the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m observed per unit area in the R 1 /2 part of the transverse cross section is made 500 pieces/mm 2 or higher.
  • the number density of sulfides is preferably 800 pieces/mm 2 or higher.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar is greatly affected by the solidifying condition during casting of the steel, the heating condition during subsequently rolling of the steel bar, or the heating condition during blooming and rolling of the steel bar. Concerning the solidifying condition, specifically, as the cooling rate from solidification start to solidification finish increases, the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar can be increased.
  • the cooling rate from solidification start to solidification finish can be estimated by using the following formula described in Non-patent Document 1 by cutting a test specimen out of the transverse cross section of cast piece or ingot and by measuring the secondary arm space of dendrite.
  • the cooling rate from solidification start to solidification finish thus estimated is preferably made 35°C/min or higher.
  • S 710 ⁇ R - 0.39
  • S is a space ( ⁇ m) between secondary dendrite arms at the middle position between the center and the surface of cast piece or ingot
  • R is an average cooling rate (°C/min) from solidification start to solidification finish.
  • the casting rate has only to be made 0.3 to 1.2 m/min, for example, when a 300 mm x 400 mm cast piece is produced by continuous casting.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar 500 pieces/mm 2 or higher in the process in which the rolled steel bar is produced by using the cast piece or ingot cast under this condition it is preferable to avoid heating at a temperature of 1300°C or higher at the heating stage of blooming and steel bar rolling.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar is affected by the heating condition during blooming and rolling of the steel bar.
  • the internal structure of the hot-forged section material has to be made a ferrite/pearlite structure. If bainite or martensite is recognized in the micro-structure, the fracture toughness value and machinability are decreased remarkably.
  • the structure after hot forging has to be refined.
  • the average pearlite grain size in the R 2 /2 part or the T/4 part of the transverse cross section of the section material has to be made 150 ⁇ m or smaller. If the average pearlite grain size is larger than 150 ⁇ m, the fracture toughness value decreases remarkably.
  • the through hole is formed by cutting work in the center part of the hot-forged section material when the common rail is produced, the machinability of the center part of the section material has to be good.
  • the machinability of the center part is greatly affected by the micro-structure in addition to the chemical composition.
  • the area fraction of pearlite accounting for the micro-structure of the center part is more than 75%, the hardness is increased remarkably, and thereby the machinability is decreased greatly. Therefore, the area fraction of pearlite accounting for the micro-structure of the center part of the hot-forged section material is made 75% or less.
  • the area fraction of pearlite accounting for the micro-structure of the center part is less than 20%, a tear or the like sometimes occurs during cutting work. Therefore, the area fraction of pearlite accounting for the micro-structure of the center part of the hot-forged section material is preferably made 20% or more.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar is made 500 pieces/mm 2 or higher.
  • the rolled steel bar for hot forging defined in the present invention is forged, it is preferable that heating at a temperature of 1280°C or higher be avoided, and that the average cooling rate from 800°C to 550°C after hot forging be made 70°C/min or lower.
  • a rolled steel bar for hot forging and a hot-forged section material that have an excellent transverse fatigue strength and a high fracture toughness value can be obtained.
  • the heating temperature at the time when the rolled steel bar for hot forging or the hot-forged section material is produced indicates the atmospheric temperature in a furnace
  • the rolling temperature and the forging temperature indicate the surface temperature of a steel material being worked.
  • the molten steels were continuously cast at a casting rate of 0.7 m/min by using a continuous casting facility, whereby cast pieces each having a transverse cross section of 300 mm ⁇ 400 mm were prepared.
  • the 300 mm ⁇ 400 mm cast pieces of steels A1 to A30 obtained by the above-described method were heated at 1250°C for 120 minutes, and thereafter were turned into 180 mm ⁇ 180 mm slabs by blooming. Subsequently, the slabs were heated at 1200°C for 90 minutes, and rolled steel bars each having a diameter of 50 mm were formed in the temperature range of 1100 to 1000°C.
  • the total reduction ratio (S O /S F ) from the cast pieces of steels A1 to A30 to the rolled steel bars was 61.
  • a specimen having a transverse cross section of 10 mm ⁇ 10 mm was cut out of the R 1 /2 part of the rolled steel bar, and resin embedding and mirror polishing were performed so that the transverse cross section was a surface to be inspected.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m was examined.
  • the magnification of a scanning electron microscope (SEM) was made ⁇ 1000, the observation region of a total area of 1.57 mm 2 in a total of 128 visual fields was photographed by backscattered electron image, and thereby the number of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m observed in the observation region was measured. The measured number of sulfides was converted into the number per unit area (mm 2 ).
  • Table 2 gives the measurement results of the predicted maximum width of nonmetallic inclusions of the rolled steel bar obtained by extreme value statistical processing and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar.
  • the "predicted maximum inclusion width” in Table 2 means the predicted maximum width of nonmetallic inclusions of the rolled steel bar
  • the "sulfide number density” means the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar.
  • the 50-mm diameter rolled steel bar obtained by rolling as described above was cut to a length of 180 mm, being reheated to 1250°C, and was subjected to hot forging in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of the rolled steel bar in the temperature range of 1200 to 1150°C. Thereby, the rolled steel bar was finished into a hot-forged section material having a thickness of about 35 mm and a width of about 60 mm.
  • the hot-forged section material was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 30°C/min.
  • the hot-forged section material having a thickness of about 35 mm and a width of about 60 mm
  • ten specimens each having a longitudinal cross section measuring 5 mm thick ⁇ 15 mm long including the T/4 part of section material were cut out of a 1/2 position of width of about 60 mm, and resin embedding and mirror polishing were performed so that the longitudinal cross section was a surface to be inspected.
  • the predicted maximum width of nonmetallic inclusions was estimated.
  • a specimen having a transverse cross section of 10 mm ⁇ 10 mm including the T/4 part of section material were cut out of a 1/2 position of width of about 60 mm. Then, after resin embedding and mirror polishing had been performed so that the transverse cross section was a surface to be inspected, the surface to be inspected was etched with alcohol containing 3% of nitric acid (nital etching reagent), whereby the micro-structure was caused to appear. Subsequently, a micro-structure image was photographed in five visual fields with the magnification of the optical microscope being x200, and thereby the "phase" in the T/4 part was identified.
  • an average pearlite grain size was calculated by arithmetically averaging the pearlite grain sizes in the five visual fields.
  • a pearlite colony group surrounded by ferrite was made a pearlite grain, and the diameter of circle corresponding to the area thereof, that is, the circle-equivalent diameter was made a pearlite grain size.
  • a specimen having a transverse cross section of 10 mm ⁇ 10 mm was cut out of the center part of the section material. Then, after resin embedding and mirror polishing had been performed so that the transverse cross section was a surface to be inspected, the surface to be inspected was etched with alcohol containing 3% of nitric acid (nital etching reagent), whereby the micro-structure was caused to appear. Subsequently, a micro-structure image was photographed in five visual fields with the magnification of the optical microscope being x200. Thereby, by using the photographed image, the area fraction of pearlite accounting for the micro-structure of the center part of the section material was determined by image processing software, and the arithmetic mean value of five visual fields was made the pearlite area fraction of the center part.
  • a No.14A test specimen (diameter of parallel part: 5 mm) specified in JIS Z 2241 (2011) was sampled so that the longitudinal direction of the test specimen was the width direction of the section material, that is, the direction perpendicular to the center axis of the section material, and the center of the parallel part of test specimen was the 1/2 position of the width of about 60 mm of the section material. Then, a tension test was conducted at room temperature with the gage length being 25 mm, and thereby the tensile strength was determined. The target tensile strength of the section material was 900 MPa or higher.
  • Both the ends in the width direction of the hot-forged section material having a thickness of about 35 mm and a width of about 60 mm were descaled by milling, and were finished into flat surfaces.
  • Both of the milled ends of the section material and a carbon steel S10C specified in JIS G 4051 (2009) were welded to each other by electron beam welding, and thereby a plate material having a thickness of about 35 mm and a width of 130 mm was prepared. Subsequently, from the T/4 part of the plate material, an Ono type rotating bending test specimen of No.
  • test piece (diameter of parallel apart: 8 mm, length of parallel apart: 17 mm, diameter of gripping part: 15 mm, R of a part between parallel part and gripping part: 24 mm, overall length: 106 mm) specified in JIS Z 2274 (1978) was prepared so that the longitudinal direction of the test specimen was the width direction of the plate material, that is, the direction perpendicular to the center axis of the section material, and the center of the parallel part of test specimen was the 1/2 position of the width of 130 mm of the plate material.
  • test specimen (length: 115 mm, width: 25 mm, thickness: 12.5 mm) specified in ASTM E 399-06 was sampled so that the longitudinal direction of the test specimen was the center axis direction of the section material, and the center of the width of test specimen was the 1/2 position of the width of about 60 mm of the section material.
  • a notch having a length of 10.5 mm (the length was constant in the test specimen width direction) was formed in the width direction at the center position in the longitudinal direction of the test specimen, and at the front end of the notch, a pre-crack having a length of 2.0 mm was introduced by fatigue load.
  • the shape of test specimen is shown in Figure 2 .
  • a clip gage was attached to the notch end part of this test specimen so that the opening displacement of notch can be measured. Then, a three-point bending load was applied to the test specimen, that is, a load was applied from the end face on the opposite side just above the notch by supporting the end face on the test specimen notch side at two points with a span of 100 mm. At this time, the load and the change of opening displacement were measured, and from the graph showing the relationship between the both, the load P Q and the maximum load P max , which were the bases of the calculation of fracture toughness value, were determined in conformity to ASTM E 399-06.
  • the target fracture toughness value K Q was 40 MPa ⁇ m 1/2 or higher.
  • the whole surface of hot-forged section material having a thickness of about 35 mm and a width of about 60 mm was descaled by milling and was finished into a flat surface. Then, after a prepared hole having a depth of 10 mm and a diameter of 9.6 mm had been formed in advance in the center part of the section material, by using a cemented carbide drill formed with a 9.5-mm diameter TiAlN-coated oil hole, piercing was performed to a depth of 90 mm per one hole.
  • a water-soluble cutting lubricating oil was supplied with the rotating speed of drill being 2011 rpm (cutting speed: about 60 m/min), with the feed per one revolution being 0.10 mm/rev, and with the oil pressure being 2 MPa.
  • the machinability was evaluated by measuring the thrust resistance by using a tool dynamometer, which thrust resistance was imparted to the center axis direction of drill when piercing was performed.
  • the machinability was evaluated by the mean value of thrust resistances measured when 10 holes were pierced.
  • the target machinability was such that the mean value of thrust resistances was 1800 N or smaller.
  • the index of machinability evaluation the material in which the mean value of thrust resistances was 1800 N or smaller was judged to be acceptable "O", and the material in which the mean value of thrust resistances was larger than 1800 N was judged to be unacceptable " ⁇ ".
  • Table 3 collectively gives the test results.
  • the "predicted maximum inclusion width” in Table 3 means the predicted maximum width of nonmetallic inclusions of the section material.
  • steels A1 to A22 used each had the chemical composition within the range of chemical composition defined in the present invention, and each had the predicted maximum width of nonmetallic inclusions and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m within the ranges of these values of the rolled steel bar defined in the present invention, all of the tensile strength, transverse fatigue strength, fracture toughness value, and machinability of the hot-forged section material exhibited excellent property values.
  • test No. 23 although the chemical composition of steel A23 used was within the range defined in the present invention, the value of Fn1 was as small as 0.80, being smaller than the value defined in the present invention, so that the tensile strength of the hot-forged section material was as low as 842 MPa, and the transverse fatigue strength thereof was as low as 400 MPa.
  • test No. 24 although the chemical composition of steel A24 used was within the range defined in the present invention, the value of Fn1 was as large as 1.24, being larger than the value defined in the present invention, and bainite was recognized in the hot-forged section material, so that the fracture toughness value was as low as 37 MPa ⁇ m 1/2 , and the value of thrust resistance was larger than 1800 N.
  • the content of Mn in steel A25 used was as high as 1.65%, being higher than the upper limit value defined in the present invention, and bainite was recognized in the section material, so that the fracture toughness value was as low as 38 MPa ⁇ m 1/2 , and the value of thrust resistance also was larger than 1800 N.
  • the content of S in steel A26 used was as low as 0.004%, being lower than the value defined in the present invention, so that the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar was as low as 255 pieces/mm 2 . Therefore, the average pearlite grain size of the section material became large, being 258 ⁇ m, and the fracture toughness value was as low as 38 MPa ⁇ m 1/2 .
  • the content of S in steel A27 used was as high as 0.049%, being higher than the value defined in the present invention, so that the predicted maximum width of nonmetallic inclusions of the rolled steel bar was as large as 109 ⁇ m. Therefore, the transverse fatigue strength of the section material was as low as 420 MPa.
  • the content of Ti in steel A29 used was as high as 0.053%, being higher than the value defined in the present invention. Therefore, the fracture toughness value of the section material was as low as 35 MPa ⁇ m 1/2 .
  • the content of O in steel A30 used was as high as 0.0045%, being higher than the value defined in the present invention, so that the predicted maximum width of nonmetallic inclusions of the rolled steel bar was as large as 132 ⁇ m. Therefore, the transverse fatigue strength of the hot-forged section material was as low as 400 MPa.
  • a small piece having a transverse cross section measuring 15 mm thick ⁇ 15 mm wide was cut out of a position of 1/4 of thickness 300 mm and 1/2 of width 400 mm of the prepared cast piece.
  • the structure was caused to appear by using a picric acid etching reagent.
  • the dendrite structure was observed under an optical microscope, and the dendrite secondary arm space was measured.
  • the secondary arm space of dendrite was measured by using calipers, and the actual dimension was determined by dividing the measured space by the photographing magnification of the photograph.
  • the dendrite secondary arm space was about 142 ⁇ m, and the cooling rate from solidification start to solidification finish was about 62°C/min.
  • the molten steel in which the chemical composition had been controlled and from which oxides had been removed by performing 90-minute treatment by using a ladle refining furnace equipped with vacuum degassing equipment (LFV), was solidified by being cast in a mold made of refractory, whereby an ingot having a height of 2000 mm, a cross section of 500 mm ⁇ 500 mm at the 1/2 position of the height of 2000 mm, and a weight of about 3.5 tons was prepared.
  • LUV vacuum degassing equipment
  • the dendrite secondary arm space was about 235 ⁇ m, and the cooling rate from solidification start to solidification finish was about 17°C/min.
  • the total reduction ratio (S O /S F ) from the cast piece to the rolled steel bar of steel B1 was 61
  • the total reduction ratio (S O /S F ) from the ingot to the rolled steel bar of steel B2 was 127.
  • the examination results are given in Table 5.
  • the "predicted maximum inclusion width” in Table 5 means the predicted maximum width of nonmetallic inclusions of the rolled steel bar, and the “sulfide number density” means the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar.
  • the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m was 1063 pieces/mm 2 , being not lower than 500 pieces/mm 2 ; in contrast, for the rolled steel bar of test No. 32, the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m was 368 pieces/mm 2 , being lower than 500 pieces/mm 2 .
  • each of the 50-mm diameter rolled steel bars was cut to a length of 180 mm.
  • the rolled steel bar was finished into a hot-forged section material having a thickness of about 35 mm and a width of about 60 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1200 to 1150°C, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 30°C/min.
  • Figure 3 shows the optical microphotographs of micro-structures in a T/4 part at the 1/2 position of the width of about 60 mm of each of section materials of test Nos. 31 and 32, which micro-structures were observed by the method described in (D) of Example 1.
  • the predicted maximum width of nonmetallic inclusions, micro-structure, tensile strength, transverse fatigue strength, fracture toughness value, and machinability were examined by the testing methods described in (C) to (H) of Example 1. The obtained results are given in Table 6.
  • the "predicted maximum inclusion width” in Table 6 means the predicted maximum width of nonmetallic inclusions of the section material.
  • the chemical compositions of steel B1 and steel B2 were within the range defined in the present invention, and were almost equivalent to each other; however, the number densities of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bars used are different. It is found that, for the rolled steel bar of test No. 32, the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m was 368 pieces/mm 2 , being lower than 500 pieces/mm 2 , and therefore the average pearlite grain size of the section material was 215 ⁇ m exceeding 150 ⁇ m, being larger than the grain size of 43 ⁇ m of test No. 31, so that the micro-structure was coarse. As the result, the hot-forged section material of test No. 32 was poor in fracture toughness value.
  • rolled steel bars for hot forging having a diameter of 50 mm or a diameter of 80 mm were produced under the conditions given in Table 7.
  • the "blooming heating condition” in Table 7 means the heating temperature for performing blooming
  • the "steel bar heating temperature” means the heating temperature for performing steel bar rolling
  • the "steel bar rolling size” means the diameter of rolled steel bar produced by steel bar rolling.
  • the predicted maximum width of nonmetallic inclusions and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m were examined by the methods described in (A) and (B) of Example 1, respectively.
  • the examination results are given in Table 8.
  • the "predicted maximum inclusion width” in Table 8 means the predicted maximum width of nonmetallic inclusions of the rolled steel bar
  • the "sulfide number density” means the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m of the rolled steel bar.
  • each of the 50-mm diameter rolled steel bars was cut to a length of 180 mm.
  • the rolled steel bar was finished into a hot-forged section material having a thickness of about 35 mm and a width of about 60 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1200 to 1150°C, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 30°C/min.
  • test No. 36 an 80-mm diameter rolled steel bar was cut to a length of 180 mm. After being reheated to 1250°C, the rolled steel bar was finished into a hot-forged section material having a thickness of about 50 mm and a width of about 100 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1200 to 1150°C, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 15°C/min.
  • steel A12 had the chemical composition within the range of chemical composition defined in the present invention, and had the predicted maximum width of nonmetallic inclusions and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m within the ranges of these values of the rolled steel bar defined in the present invention, all of the predicted maximum width of nonmetallic inclusions, tensile strength, transverse fatigue strength, fracture toughness value, and machinability of the section material exhibited excellent property values.
  • test Nos. 34 and 35 although the chemical composition of steel A12 used was within the range defined in the present invention, the number densities of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m were 470 pieces/mm 2 and 359 pieces/mm 2 , respectively, being lower than the range defined in the present invention. Therefore, the average pearlite grain sizes of the section materials were 235 ⁇ m and 186 ⁇ m, respectively, being larger than 150 ⁇ m, and the fracture toughness values were as low as 38 MPa ⁇ m 1/2 and 39 MPa ⁇ m 1/2 , respectively.
  • the 50-mm diameter rolled steel bar was cut to a length of 180 mm. After being reheated to 1250°C, the rolled steel bar was formed into a section material having a thickness of about 35 mm and a width of about 60 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1200 to 1150°C, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 30°C/min.
  • the 50-mm diameter rolled steel bar was cut to a length of 180 mm. After being reheated to 1290°C, the rolled steel bar was formed into a section material having a thickness of about 35 mm and a width of about 60 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1250 to 1200°C, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 30°C/min.
  • the 50-mm diameter rolled steel bar was cut to a length of 180 mm. After being reheated to 1250°C, the rolled steel bar was formed into a section material having a thickness of about 35 mm and a width of about 60 mm by being subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction of rolled steel bar in the temperature range of 1200 to 1150°C, and was cooled to room temperature by being fan-cooled. The cooling rate in the temperature range of 800 to 550°C was approximately 90°C/min.
  • Table 10 Test No Steel No. Predicted maximum width of nommetallic inclusions Micro-structure Average pearlite grain size Area fraction of pearlite of the center part Tensile strength Fatigue strength Fracture toughness K Q Thrust resistance ( ⁇ m) ( ⁇ m) (%) (MPa) (MPa) (MPa ⁇ m 1/2 ) 37 A13 34 F+P 45 58 985 465 57 ⁇ 38 A13 33 F+P * 175 * 80 1035 470 38 ⁇ 39 A13 36 * F+P-B - - 1051 480 39 ⁇ * indicates that conditions do not satisfy those defined bv the present invention.
  • steel A13 had the chemical composition within the range of chemical composition defined in the present invention, and had the predicted maximum width of nonmetallic inclusions and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m within the ranges of these values of the rolled steel bar defined in the present invention, and also since the predicted maximum width of nonmetallic inclusions of the section material and the micro-structure within the ranges defined in the present invention, all of the tensile strength, transverse fatigue strength, fracture toughness value, and machinability exhibited excellent property values.
  • test No. 38 although the chemical composition was within the range of chemical composition defined in the present invention, and the predicted maximum width of nonmetallic inclusions and the number density of sulfides each having a circle-equivalent diameter of 0.3 to 1.0 ⁇ m were within the ranges of these values of the rolled steel bar defined in the present invention, since the average pearlite grain size in the T/4 part of the transverse cross section of the section material and the pearlite area fraction in the center part deviated from the range defined in the present invention, the fracture toughness value and machinability were poor.
  • Steel C1 is a steel material corresponding to SCM435 specified in "Low-alloyed Steels for Machine Structural Use" of JIS G 4053 (2008).
  • the 300 mm ⁇ 400 mm cast piece of steel C1 was heated at 1250°C for 120 minutes, and thereafter a slab measuring 180 mm ⁇ 180 mm was produced by blooming. Subsequently, the slab was heated at 1200°C for 90 minutes, and was rolled into a steel bar in the temperature range of 1100 to 1000°C, whereby a rolled steel bar having a diameter of 50 mm was produced.
  • the total reduction ratio (S O /S F ) from the cast piece to the rolled steel bar of steel C1 was 61.
  • each of the 50-mm diameter rolled steel bars for hot forging of steel A12, steel A14, and steel C1 was cut to a length of 250 mm, thereafter being reheated to 1250°C, and was subjected to hot forging, in which the rolled steel bar was pressed down in the direction perpendicular to the rolling direction in the temperature range of 1200 to 1150°C, whereby a common rail-shaped hot-forged section material shown in Figure 4 was produced, and was cooled to room temperature by being allowed to cool in the atmosphere.
  • the cooling rate in the temperature range of 800 to 550°C was approximately 45°C/min.
  • the hot-forged section material for common rail was produced by integral molding, and was configured by a shell part 1, which is a common rail body, and five branch parts 2a to 2e.
  • the outside diameter of the shell part 1 was 30 mm.
  • the "predicted maximum inclusion width" in Table 12 means the predicted maximum width of nonmetallic inclusions of the section material. As shown in Figure 4 , on the common rail-shaped section material, the predicted maximum width was determined by taking the width of nonmetallic inclusion in the R 2 /2 part (R 2 : radius of the shell part 1) of the longitudinal cross section of the shell part 1, that is, at position 7.5 mm deep from the surface as W ( ⁇ m).
  • the pearlite area fraction of the center part of section material was calculated in the center part of the shell part 1, and the average pearlite grain size was measured in the R 2 /2 part (R 2 : radius of the shell part 1) of the transverse cross section of the shell part 1, that is, at position 7.5 mm deep from the surface.
  • a through hole 11 was formed in the center axis direction in the center part thereof by cutting work, and minute holes 12a to 12e were formed in the five branch parts 2a to 2e by cutting work so as to intersect with the through hole, whereby a common rail having the shape shown in Figure 5 was produced.
  • Figure 5(a) is a front view
  • Figure 5(b) is a side view.
  • the cutting work was performed by using a gun drill under the conditions that the cutting speed was 70 m/min and the feed per one revolution was 0.03 mm/rev.
  • oil quenching was performed by heating at 870°C for 60 minutes, and successively tempering was performed at 600°C for 90 minutes.
  • a fatigue test was conducted.
  • a pressure generating source was connected to the minute hole 12a formed in the branch part 2a of the five branch parts, and a pressure sensor was provided in an intermediate location between the minute hole and the pressure generating source. All of the end portions of other minute holes 12b to 12e and both the ends of the through hole 11 formed in the shell part 1 were sealed. Subsequently, oil was supplied under pressure from the minute hole 12a connected to the pressure generating source so that the stress is fluctuated periodically (frequency: 15 Hz). The maximum pressure at endurance of number of cycles of 1.0 ⁇ 10 7 or larger was made the fatigue strength. The ratio with respect to test No. 42 was determined as a fatigue limit ratio, and evaluation was performed. The pressure was an internal pressure measured by the pressure sensor installed between the pressure generating source and the minute hole 12a in the end portion of common rail. The test results are given in Table 13.
  • a non-thermally refined hot-forged section material excellent in transverse fatigue strength, fracture toughness value, and machinability can be obtained. Also, by forming intersecting holes in the hot-forged section material of the present invention, a common rail for a fuel injection system used at a high injection pressure can be produced at a low cost.

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  • Chemical & Material Sciences (AREA)
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  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
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EP13749063.7A 2012-02-15 2013-02-05 Tige d'acier laminée pour forgeage à chaud, matériau grossièrement façonné forgé à chaud et rampe commune et sa procédé de fabrication Active EP2816131B1 (fr)

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PCT/JP2013/052579 WO2013121930A1 (fr) 2012-02-15 2013-02-05 Acier pour tiges laminées pour forgeage à chaud, matériau grossièrement façonné forgé à chaud et rail commun, et leur procédé de fabrication

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CN108138288A (zh) * 2015-10-19 2018-06-08 新日铁住金株式会社 热锻造用钢及热锻造品
WO2021083588A1 (fr) * 2019-10-28 2021-05-06 Robert Bosch Gmbh Composant, en particulier conduit de carburant ou distributeur de carburant, et système d'injection de carburant
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CN108138288A (zh) * 2015-10-19 2018-06-08 新日铁住金株式会社 热锻造用钢及热锻造品
EP3366799A4 (fr) * 2015-10-19 2019-06-12 Nippon Steel & Sumitomo Metal Corporation Acier pour forgeage à chaud, et article forgé à chaud
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US11111568B2 (en) 2016-09-30 2021-09-07 Nippon Steel Corporation Steel for cold forging and manufacturing method thereof
WO2021083588A1 (fr) * 2019-10-28 2021-05-06 Robert Bosch Gmbh Composant, en particulier conduit de carburant ou distributeur de carburant, et système d'injection de carburant

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US20150034049A1 (en) 2015-02-05
JP5778055B2 (ja) 2015-09-16
WO2013121930A1 (fr) 2013-08-22
EP2816131B1 (fr) 2020-05-06
CN104114734A (zh) 2014-10-22
CN104114734B (zh) 2016-06-29
US20180057917A1 (en) 2018-03-01
US9951403B2 (en) 2018-04-24
EP2816131A4 (fr) 2016-03-23
JP2013166983A (ja) 2013-08-29
US9994943B2 (en) 2018-06-12

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